The invention relates to a method for setting the filling pressure for a clutch comprising clutch linings, a clutch release spring and a hydraulic pressure system for closing the clutch against a force from the clutch release spring, wherein the clutch linings are brought at an engagement point of the clutch into abutment such that a further increase of the pressure in the pressure system would establish a torque transmittal capacity in the clutch.
Typically, the hydraulic pressure system of a clutch cylinder comprises a clutch piston that is connected to an input side and an output side of the clutch in a fixed manner and rotates therewith. At the engagement point of the clutch the force from the clutch cylinder exerted onto the clutch pads is compensated by the force from the clutch release spring so that the clutch linings just abut against each other but without exerting any significant thrust forces against each other.
In the automotive field use of clutches that are automatically controlled by a control device have become more and more common, as for instance clutches in dual clutch transmissions. For this type of actuating a clutch it is important to determine exactly the engagement point for guaranteeing high quality of the clutch operation.
For determining this engagement point starting from the entirely released clutch position, the clutch cylinder needs to be filled with hydraulic fluid. For accomplishing this filling action quickly, the filling pressure can be set to a high value. Such a high filling pressure allows establishing a high volume flow in the pressure system so that the clutch cylinder is filled quickly so that the engagement point, starting from the released clutch, is reached in a short period of time. However, due to the high filling pressure there is a risk that the clutch cylinders and the clutch piston exert a force onto the clutch lining during the filling action that is higher than the counterforce exerted by the clutch release spring, resulting in the clutch already having some torque transmittal capacity. In a motor vehicle where a clutch is filled in that manner, this may result in a noticeable impact that affects the driving comfort.
If in contrast the filling pressure is set to a value that is too low, filling of the clutch cylinder and therefore reaching the engagement point can take so much time that driving comfort is likewise affected when shifting gears in the motor vehicle.
In the DE 10 2006 045 573 published on Apr. 3, 2008 after the priority date of the present application a method for setting a filling pressure for a clutch is described wherein the hydraulic pressure system is subjected to a filling pressure that is significantly higher than the engagement pressure maintaining the hydraulic pressure system of the clutch at the engagement point. The filling by the filling pressure ends when the actual pressure has reached a predetermined turn off pressure. After reaching the turn off pressure and after a damping period a face size K between the actual pressure and the engagement pressure is calculated. Depending on the calculated face size K (negative/positive prefix) the filling pressure is adjusted for the next filling. Several filling actions are repeated until the turn off criterion is reached, for example when the determined face size K falls below a threshold value.
Although according the method described in DE 10 2006 045 573 can generally reach good results, such a method may in the practical application be problematic, in particular when the pressure in the hydraulic pressure system is controlled after reaching the turn off pressure to become the engagement point pressure. Therefore, the face size K does not provide a reliable determination of the filling status of the hydraulic pressure systems.